fusefs: add a test for the subtype= option

At mount time server can set, for example, "subtype=xfs", so that
mount(8) will later show the mountpoint's file system as "fusefs.xfs".
fusefs has had this feature ever since the original GSoC commit in 2012,
but there's never been a test for it.

(cherry picked from commit 616eaa66aa4955f182eb0117e17e07da76fcf696)